1. Field of the Invention
The present invention relates to monopole antenna and more particularly, to a multi-band monopole antenna, which improves hearing aid compatibility performance by means of an application of interference theory.
2. Description of the Related Art
Following fast development of wireless communication technology, many communication devices, such as cell phone, PDA and etc. employ multi-band for receiving and transmitting wireless signal. Therefore, many standards, including GSM (Global System for Mobile Communications), DCS (Distributed Control System), PCS (Personal Communication Service, AMPS (Advanced Mobile Phone System), PDC (Personal Digital Cellular), CDMA (Code Division Multiple Access), etc. are established. Under the compact requirement for handheld devices, antenna structure is required to be space-saving. In consequence, a monopole antenna is designed to be mounted inside the casing of a handheld device.
For example, U.S. Pat. No. 7,405,701, entitled “Multi-band bent monopole antenna” discloses a wireless communication device using a multi-band antenna. The multi-band antenna comprises a main antenna element and a parasitic element disposed proximate a portion of the main antenna element. Further, a selection circuit selectively applies capacitive coupling to the multi-band antenna to improve the bandwidth of a first frequency band without adversely affecting the bandwidth of a second frequency band. When the multi-band antenna operates in a low frequency band, the selection circuit fails to apply capacitive coupling to the multi-band antenna.
Further, conventional multi-band monopole antennas show a poor result in HAC (hearing aid compatibility) performance. HAC (hearing aid compatibility) regulations for antenna in cell phone have been established. ANSI (American National Standards Institute established ANSI C63.19 that establishes compatibility between hearing aids and cellular telephones. To ensure that sufficient hearing aid-compatible digital wireless phones complying with the ANSI standard are available, the FCC in 2008 set benchmark date by which digital wireless handset manufacturers and service providers had to increase the number of hearing aid-compatible digital wireless phones available to consumers to the 50 percent threshold by Feb. 18, 2008.
Similar to U.S. Pat. No. 7,405,701, FIGS. 1 and 2 show another prior art design of multi-band monopole antenna 91. According to this design, the multi-band monopole antenna 91 is installed by means of connecting its feed end 911 connected to a PC board 92. The antenna 91 has the dimension of 37 mm×18 mm×8 mm. The PC board 92 has a height 2 mm, a length 110 mm and a width 50 mm. Further, the clearance zone on the PC board 92 is 21 mm×50 mm. FIG. 3 shows the distribution of electric field and magnetic field under a low frequency band (900 MHz) of HAC testing on the multi-band monopole antenna 91. FIG. 4 shows the distribution of electric field and magnetic field under a high frequency band (1800 MHz) of HAC testing on the multi-band monopole antenna 91. From the high frequency band test result shown in FIG. 4, this prior art design of multi-band monopole antenna has room for improvement on HAC.